1. Field of the Invention
This invention relates to bypass valves for use in wellbores, particularly but not exclusively to the secondary means for closing a bypass valve in the event that the primary means for closing the bypass valve fails to operate.
2. The Prior Art
It is common practice in the oil and gas drilling industry to incorporate a bypass valve in a drill string between a MWD (Measurement While Drilling) tool and a hydraulic anchor packer so that wellbore fluid may be pumped down the drill string to operate the MWD tool without prematurely setting the anchor packer. A conventional bypass valve typically incorporates a piston which slides within a cylinder in response to dynamic fluid pressure. The wall of the cylinder is provided with a plurality of holes which allows fluid to pass from the drill string bore to the wellbore annulus. The piston is held in an open position by biassing means (such as a spring or a shear pin) and thereby allows wellbore fluid to operate a MWD tool located uphole of the bypass valve whilst preventing the generation of a pressure differential between the interior and exterior of the drill string sufficient to set an anchor packer. When the setting of the anchor packer is required, the flow of wellbore fluid down the drill string is increased so as to generate a dynamic pressure sufficient to overcome the biassing means. The piston then slides within the cylinder to a closed position in which the holes are sealed. A cross-sectional side view of this type of bypass valve is shown in FIG. 1.
Conventional bypass valves can occasionally fail to move to the closed configuration when the appropriate fluid pressure is applied and this will often lead to costly and time consuming delays in a given downhole operation. In an attempt to overcome this problem, a "sliding piston" type of bypass valve (such as the one described above) has been developed with a secondary closing means in addition to the primary closing means (the sliding piston). A cross-sectional side view of this improved bypass valve is shown in FIG. 2. In the event that the primary piston within the cylinder fails to move to the closed position in response to an increase in dynamic pressure, the static pressure of the wellbore fluid in the annulus may be increased by a pump located at the surface, with the internal bore of the drill string having been sealed off, so as to generate a sufficient pressure above the downhole hydrostatic pressure to rupture a burst disc provided in the bypass valve casing. A pressure differential is thereby applied across the length of a second piston and acts to press the second piston into a closed position. The location of the second piston in the closed position is such that the holes in the wall of the cylinder are sealed. Thus, although the primary closing means may fail to operate correctly, the bypass valve can nevertheless be moved into a closed configuration by the operation of the secondary closing means.